Family members wearing estelada or independence flags on their back walk at the end of a big rally during the Catalan National Day in Barcelona, Spain, Monday Sept. 11, 2017. Some thousands rally in Barcelona to show support for an independent Catalan nation and the right to vote in a controversial referendum that has been banned by Spain. (AP Photo - Francisco Seco)

From Associated Press

September 13, 2017 9:26 AM EST

MADRID (AP) — Spain's top prosecutor is investigating more than 700 Catalan mayors for cooperating with a planned referendum on the region's independence after the nation's constitutional court ordered the vote put on hold, the prosecutor's office said Wednesday.

Catalonia's regional police force is under orders to arrest the mayors if they refuse to appear for questioning, State Prosecutor Jose Manuel Maza's office said.

The announcement significantly raised the stakes in an increasingly tense standoff between Catalan independence supporters and national authorities over the referendum planned for Oct. 1.

If mayors and their municipalities cannot help organize balloting, the vote is unlikely to proceed. Maza's order also puts regional police officers in the uneasy position of carrying out commands from Madrid in their towns and cities.

The pro-independence coalition ruling Catalonia has vowed to hold the referendum, defying a prohibition by Spain's Constitutional Court. It has asked the 947 mayors in the northeastern region to provide voting facilities.

Maza ordered the prosecutors in Spain's 17 provinces to investigate the 712 mayors who already have offered to provide municipal premises as polling stations.

Most of Catalonia's mayors have said they would cooperate with the referendum. However, the willing mayors represent less than half of the region's voting-age population.

Urban support is key for the pro-independence movement, especially the Catalan capital of Barcelona, which is home to around 20 percent of voters.

Barcelona Mayor Ana Colau, who opposes secession but supports a vote, says she wants to help arrange the referendum but won't do so without assurances that she and her staff would be acting legally.

Such assurance is unlikely to materialize, and without Barcelona's participation, the referendum would lack legitimacy.

Prime Minister Mariano Rajoy appealed to Catalans to ignore calls from independence supporters to turn out to vote.

"If anyone urges you to go to a polling station, don't go, because the referendum can't take place, it would be an absolutely illegal act," Rajoy said.

Spain's King Felipe VI also entered the fray, stepping up the pressure on Catalonia by vowing that the Spanish Constitution "will prevail" over any attempt to break the country apart.

In his first comments on the growing political crisis, Felipe said the rights of all Spaniards will be upheld against "whoever steps outside constitutional and statutory law."

Copyright 2017 The Associated Press. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.